The present invention relates to laser interferometric method and apparatus with large collecting aperture for measuring motion from a surface. The invention is particularly directed toward detecting small displacement of a surface, and more particularly, an optically scattering surface, of a workpiece subjected to ultrasound, where the workpiece does not need to be accurately positioned.
The detection of phase modulation of an optical wave is important for various field of application where optical beams are used to detect the small motion of an object. This is the case of laser detection of ultrasound and transient body motion such as those produced by the sudden absorption of a laser pulse. The technique based on laser generation and detection, known as Laser Ultrasonic (LU) or Laser Based Ultrasonic (LUB), allows for remote and non-contact detection and generation of ultrasound. Laser ultrasonics can thus be advantageously used for inspection at high temperature, on-line and during process. Most of the industrial application of laser ultrasonics involves the inspection of an “optically rough” surface. The probed surfaces being optically rough, the ultrasonic information is thus encoded in an optical beam with speckle. A suitable interferometric technique should integrate effectively over the large speckle field and provide an output signal independently of the speckle nature the collected light. Adequate sensitivity requires an interferometric technique which has a large effective light gathering efficiency. Interferometric techniques based on plane wave reference using heterodyne scheme (U.S. Pat. No. 4,633,715) or quadrature scheme (U.S. Pat. No. 5,404,224) are only effective if the two interfering beams (the reference beam and the object beam reflected from the sample) are plane waves. For object with rough surfaces reflecting a speckled beam, the plane-wave interference requirement means that only a single speckle should be selected from the object beam for high efficiency interference. Single speckle selection means that the collecting aperture is very small and thus these interferometers have very low sensitivity on rough surfaces. Instead of single speckle detection, these interferometers can be optimized for collecting few speckles in order to increase the amount of collected light, but unfortunately the sensitivity is not directly improved by the higher amount of collected light because the random phase distribution of speckles reduces the interference efficiency.
In various U.S. patents, laser interferometric schemes for detection on rough surfaces have been described. These schemes are characterized by their large étendue parameter. These laser interferometric schemes are based on confocal Fabry-Pérot (U.S. Pat. No. 4,966,459) or adaptive photorefractive interferometer (U.S. Pat. No. 5,131,748)
For industrial application, the reduction in the need for accurate object positioning is as critical as to have a high sensitivity on a rough surface. In most industrial application, the positioning of the sample is not controlled accurately. An optical system with high sensitivity but high requirement for target positioning will lead to an overall system with poor sensitivity, where good signals are acquired only sporadically when the sample moves through the optimum position. To better control the sample position may not be easily possible or may highly increase the overall cost of the inspection system, making the system not affordable and limiting its field of application. Often a compromised is used by detuning the system in order to get more uniform performance over a larger range of sample positions, at the expense of the maximum sensitivity.
Accordingly, it is one of the principle objects of the invention to provide a method and apparatus for optically detecting ultrasonic transient motion from scattering surface, having a high sensitivity, a large gathering optics as well as a large depth-of-field. The depth-of-field (DOF) is defined as the variation in the stand-off distance that causes a reduction by a factor two from the maximum sensitivity of the optical system.